The typical image sensor in digital cameras includes a two-dimensional array of light sensors, each corresponding to one pixel of an image. Each light sensor develops an electrical charge in response to light. The brighter the light and the longer the exposure, the more charge is built up. Image data is generated by digitizing and reading out the charges of all the light sensors in the image array. After generating the image data, the sensors are reset by passing the built-up electrical charges to ground, which is referred to as resetting the sensors.
Typical exposure times for film-based and digital cameras having mechanical shutters for still images of well-lit scenes can extend from 1/250th of a second to 1/60th of a second. However, there are many instances where a longer exposure time is desirable to highlight movement, or to capture a particular scene or event. An exposure time of one-half second can emphasize the speed of an object in relation to its surroundings. If a scene has low lighting conditions, a longer exposure time can enhance dim objects in the scene.
In the case of film-based cameras as well as digital cameras with mechanical shutters, an image having a long exposure is created by simply opening a mechanical shutter for an extended period of time, as selected by the user or automatically in response to lighting conditions. In digital cameras, the charge present in each light sensor is digitized after the exposure period, i.e., when the shutter closes. However, many lower-cost cameras and video cameras do not have a mechanical shutter and instead rely on a rolling shutter, which is an implementation of an electronic shutter.
In an electronic shutter, releasing a light sensor from reset, thereby allowing the corresponding electrical charge to build up, is equivalent to opening a mechanical shutter for that sensor. In a rolling shutter, multiple rows may be released from a reset condition at a time. While some rows are “exposed” by releasing them from reset, image data is read from a previously exposed row of sensors. After the last row of sensors is read, the process may be repeated to capture a new image frame.
For cameras having electronic shutters, exposure times may be varied by changing the number of rows of sensors that are exposed at any one time and by modifying the read-out clock speed, which affects the speed with which each successive row of sensors is read-out. The faster the read-out clock speed, the less time it takes a row of pixels to be read-out, and the sooner successive rows are read-out. This results in less exposure time for each row of sensors.
To satisfy the insatiable desire to provide new and better features in electronic imaging devices, it would be desirable to allow a user to enhance the use of the electronic shutter by providing imaging features not possible with a mechanical shutter, such as imaging some regions of an image with a longer exposure time, and other regions with a shorter exposure time.